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I'm trying to develop a replacement for a BLDC motor driver board and since my first attempt resulted in a few burned components now I'm trying to use the same components they used on the original board but I need help identifying a diode.

The board uses 3 FAN7382 as FET drivers and the schematic on the datasheet shows a diode connected in series with a resistor, labeled DBOOT and RBOOT respectively, but there is no reference to it in the datasheet.

Since I'm not experienced with BLDC motor control I have no idea of the required characteristics for this diode neither I can identify it by the marking (S1) on it.

Even the package is unusual, I'm unable to identify it. The approximate size of the diode is 2.50mm x 1.25mm and the forward voltage is 0.6 Volts.

You can see the diode in picture bellow.

Diode

Edit

I was able to figure out the reference of the diode. It is a Bourns SD1005-S0180, it has a reverse voltage of 80V, a forward current of 100mA and 4 ns reverse recovery time.

Link to datasheet here.

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2 Answers 2

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Ah... the good old data sheet not showing values or part numbers: -

enter image description here

Yes it is a problem when this happens and sometimes you've just got to figure it out the hard way. It's going to be a fast reverse recovery diode because Cboot is used to generate a dc voltage way above the 15 volt rail and this is being "tickled" at your PWM frequency. I've seen a lot of schottky diodes used here but seriously any fast recovery diode is going to be OK.

Possibly it's an S1 diode from vishay like this but reverse recovery is 1.8us and this strikes me a little long. Probably it's this from NXP - it does say in the data sheet that the marking is S1: -

enter image description here

Typically its reverse recovery time is sub 1us so it's probably OK.

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  • \$\begingroup\$ Thank you very much for your explanation. Is the forward current important? \$\endgroup\$
    – Bruno Ferreira
    Jun 13, 2014 at 9:21
  • \$\begingroup\$ I'd use the link in Spehro's answer for determining values. \$\endgroup\$
    – Andy aka
    Jun 13, 2014 at 9:33
  • \$\begingroup\$ On my application the high-side FET drain is connected to +24V power rail, should I consider this voltage or can the motor coils induce some spikes on the power rail that should be taken into account? \$\endgroup\$
    – Bruno Ferreira
    Jun 13, 2014 at 9:33
  • \$\begingroup\$ I'd have to read the notes on the chip and look at the reference in Spehro's answer and, as it is your circuit and you are more familiar with it and, you have a bigger motive than me I'd suggest you need to do the research. Another option is to simulate that part of the circuit in LTSpice and see how warm the diode will likely get and what peak/average currents are going to flow versus resistor size. \$\endgroup\$
    – Andy aka
    Jun 13, 2014 at 9:37
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I don't know the exact type that is, but from your description of the function, it's a bootstrap diode for the gate driver. Here is the Fairchild application note that would be used to pick a similar diode.

Something electrically similar to the ES1J-L, but in that tiny package would be appropriate.

It needs to block the upper rail voltage, so if it's a rectified-mains-voltage bus you want (really want) a 600V PIV diode.

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  • \$\begingroup\$ I had looked at a few applications notes but I missed the one you linked where the bootstrap circuit operation is very well explained. Thank you very much. \$\endgroup\$
    – Bruno Ferreira
    Jun 13, 2014 at 10:01

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